Two Rosneft workers operating an oil drilling rig in March. The Russian state-owned oil company is in a multi-billion-dollar legal dispute with Sistema, a privately owned conglomerate.CreditAndrey Rudakov/Bloomberg

MOSCOW — Sistema and its owner had for years assiduously steered clear of politics in Russia, hoping to avoid the kind of legal issues that often dog high-profile businesses here. But as it has grown into the country’s largest privately owned conglomerate, it has found itself mired in a multi-billion-dollar legal battle.

The dispute between Sistema and Rosneft, Russia’s state oil company, dates back to a 2014 nationalization, pitting two of the country’s most important companies against each other. The latest twist in their back-and-forth came on Monday, as Sistema warned banks about problems it is having repaying $66 million worth of loans.

The conflict between the two companies is another worrying sign for the investment climate in Russia. As the overall economy stagnates amid sanctions and low oil prices, the environment has raised the specter of the country’s elite fighting over a smaller pie, with businesses using courts and law enforcement that are widely seen as vulnerable to corruption to gain an advantage.

Sistema and its principal owner, Vladimir P. Yevtushenkov, have stayed away from politics — in one case, the company smoothly navigated the ouster of a former mayor, even though many of its assets are in the city of Moscow, staying aloof from the political struggle. It now employs about 150,000 people, and it says its revenues are equivalent to about 1 percent of Russia’s gross domestic product.

The conglomerate, which has assets ranging from farms to a cellphone operator, had hoped that by staying out of the political spotlight it could avoid legal entanglements. Executives seen as disloyal to the Kremlin have long been seen as targets, facing harassment and legal action. The onetime owner of the Yukos oil company, Mikhail B. Khodorkovsky, for example, was arrested after financing opposition politicians. While Mr. Yevtushenkov was placed under house arrest during Bashneft’s nationalization in 2014, because he had never dabbled in opposition politics, it was widely seen as a consequence of this business dispute.

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A worker at a Bashneft oil refinery in 2013. The dispute between the conglomerate Sistema and the Russian government began in 2014, when Bashneft, Sistema’s oil subsidiary, was seized and nationalized by the government.CreditSergei Karpukhin/Reuters

“This conflict is causing a lot of harm to the investment attractiveness of Russia,” Aleksander Y. Abramov, a professor at the Russian Academy of National Economy and Public Administration, said in a telephone interview. “This case can create a precedent when any activity in the past can be contested, and property rights put in question, for any company.”

The dispute traces its origins to Sistema’s loss of its Bashneft oil subsidiary in the 2014 nationalization. Sistema had accumulated shares in Bashneft from regional elites, who had in turn acquired it years before from a regional government.

At the time of the nationalization, officials asserted that Russia’s federal government had erred in the original transfer of Bashneft to the regional government in 1993, and voided all subsequent transactions. The state property agency then sold a controlling stake in Bashneft to Rosneft.

The state energy company is now suing Sistema for mismanaging Bashneft, and stripping it of assets, demanding $3 billion in compensation. Sistema denies the allegations.

On Monday, Sistema declared a technical default on 3.9 billion rubles, or about $66 million, in loans. The company said the technical default, a formal notice triggered by its failure to comply with some conditions on part of its debt, had occurred because a court overseeing the case had frozen Sistema assets used as collateral for the loans.

The privately owned company said that it would repay the loans, and that it did not anticipate banks demanding early repayment.

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The principal owner of Sistema, Vladimir P. Yevtushenkov, left, at a court hearing in November 2014.CreditSergei Karpukhin/Reuters

“The situation is a direct consequence of the plaintiffs, who must have known that freezing our assets would lead to a technical default,” Mikhail Shamolin, Sistema’s director, said in a statement on Monday.

In a statement issued on Monday, Rosneft’s press secretary, Michael Leontiev, suggested Sistema was using the case to mislead creditors. “If Sistema wants to use a wholly formal process, such as freezing assets, to deceive its creditors, that says a lot about the business style of this company,” he said. At Rosneft’s annual shareholder meeting last month, Igor Sechin, the chief executive, said the lawsuit against Sistema is improving Russia’s investment climate by rooting out poor corporate governance.

The conflict has rattled investors, with Sistema shares falling about 3 percent on Monday.

The company had been trying to attract international investment into the Russian economy. This year, it carried out an initial public offering for one of its subsidiaries, Detsky Mir, which operates a collection of toy stores. The I.P.O. was the first in Russia since crisis.

It has also cast a light on the struggles facing Rosneft, which cannot raise money directly from international banks because it was sanctioned after the conflict in Ukraine. The sanctions ban any long-term lending to the energy company.

In an interview last month with RBC, a Russian business newspaper, Mr. Shamolin said the case could be easier to understand, if only there was a political context.

Sistema might defend itself, he said, “if there was some sort of logic, that was discussed in the press, that the company had behaved improperly — by the unwritten rules.”

A version of this article appears in print on , on Page B4 of the New York edition with the headline: As Russian Oil Company and Private Firm Spar, Investors Worry. Order Reprints | Today’s Paper | Subscribe